Recently, a multilevel converter is quickly developed and applied to the high-voltage and large-power fields because the multilevel converter is able to obviously reduce the voltage stress of switch elements. Moreover, the benefits of the multilevel converter are obvious when a filtering inductor is operated at a high frequency and a low voltage. Consequently, the volume of the filtering inductor is decreased, and the power loss is reduced. Moreover, the voltage change rate (dv/dt) for the multilevel converter is very low; and the fluctuation of the common mode voltage is low.
The circuitry configurations of the multilevel converter are classified into three types, including a diode-clamped multilevel converter, a cascade multilevel converter and a flying-capacitor multilevel converter. In comparison with the diode-clamped multilevel converter and the cascade multilevel converter, the flying-capacitor multilevel converter has many benefits. For example, the flying-capacitor multilevel converter has simplified circuitry, less amount of components and a lot of redundant switching states. Consequently, the manufactures make efforts in developing the flying-capacitor multilevel converter.
As known, it is important to balance the voltages of the flying capacitors of the flying-capacitor multilevel converter. FIG. 1 is a schematic circuit diagram illustrating a conventional flying-capacitor multilevel converter. As shown in FIG. 1, the flying-capacitor multilevel converter comprises plural flying capacitors, even-numbered switch elements and a filtering inductor. The even-numbered switch elements are serially connected between a positive electrode and a negative electrode of a DC voltage source. The filtering inductor is connected with the middle point of the serially-connected even-numbered switch elements. A first end of each flying capacitor is connected with the two adjacent switching elements at a first side of the middle point. A second end of each flying capacitor is connected with the two adjacent switching elements at a second side of the middle point. In other words, the flying-capacitor (p+1)-level converter is composed of p basic units Ub and a filtering inductor. As shown in FIG. 1, the flying-capacitor five-level converter is defined by four basic units Ub and a filtering inductor Lf collaboratively. Each basic unit Ub comprises a flying capacitor and two switching elements. The flying capacitor is connected with the input terminals of the basic unit Ub. The two switching elements are connected with the output terminals of the basic unit Ub. For example, the first basic unit comprises the flying capacitor C1 and the two switch elements Q1 and Q1b, the second basic unit comprises the flying capacitor C2 and the two switch elements Q2 and Q2b, a third basic unit comprises the flying capacitor C3 and the two switch elements Q3 and Q3b, and a fourth basic unit comprises the flying capacitor C4 and the two switch elements Q4 and Q4b. The input terminals of the fourth basic unit (voltage input terminal/high voltage side) are electrically connected with a high voltage DC bus (not shown) to receive an input voltage V1. The output terminals of the fourth basic unit are connected with the input terminals of the third basic unit. The output terminals of the third basic unit are connected with the input terminals of the second basic unit. The output terminals of the second basic unit are connected with the input terminals of the first basic unit. Moreover, the first end and the second end of the flying capacitor of each basic unit Ub are connected with the input terminals of the basic unit Ub in parallel. The first end and the second end of the flying capacitor are respectively connected with the first ends of the two complementary switch elements at the upper side and the lower side. The second ends of the two complementary switch elements at the upper side and the lower side are connected with the input terminals of the downstream basic unit Ub. The output terminals of the first basic unit Ub are connected with an end of the filtering inductor Lf. The switch elements of all basic units Ub are selectively turned on or turned off. Consequently, plural DC levels are generated at the output terminals of the basic unit Ub (i.e., the middle point, the positive voltage output terminal or the negative voltage output terminal). After plural DC levels are subjected to a low pass filtering operation by the filtering inductor Lf and a filtering capacitor Cf, an output voltage V2 is outputted to a low voltage side.
In a conventional voltage balance control method for the flying capacitor, the charging and discharging procedures of the flying capacitor are controlled to maintain the anticipated voltage value of the flying capacitor. Take the flying-capacitor (p+1)-level converter comprising p basic units Ub as an example. From the low voltage side (i.e., the filtering inductor side) to the high voltage side (i.e., the DC power source side), the anticipated voltage of the flying capacitor of the m-th basic unit is (V1×m)/p. As shown in FIG. 1, the anticipated voltages of the flying capacitors for the first-stage, the second-stage, the third-stage and the fourth-stage basic units of the five-level converter are (V1×1)/4, (V1×2)/4, (V1×3)/4 and (V1×4)/4, respectively. Since the two switch elements at the upper side and the lower side of each basic unit Ub perform the complementary operations, the voltage withstood by each of the two complementary switch elements is the voltage difference between the present-stage flying capacitor and the next-stage flying capacitor (i.e., V1/4). Generally, by adjusting the phases and duty cycles of the switch elements to generate the plural DC levels, the voltage or current of the filtering inductor at the low voltage side is controlled. Consequently, the function of switching and regulating power can be achieved.
FIG. 2 is a schematic circuit diagram illustrating the relationship between a flying capacitor and the corresponding switch elements of the flying-capacitor multilevel converter of FIG. 1. As shown in FIG. 2, the flying capacitor C1 is connected with two serially-connected switch elements Q1, Q2 and two serially-connected switch elements Q1b, Q2b. The on/off states of the switch elements Q1 and Q2 are complementary to on/oft states of the switch elements Q1b and Q2b, respectively. By controlling the on/off states of the two adjacent switch elements, the charging/discharging procedure of the flying capacitor C1 is achieved. In the normal working state, each basic unit Ub is serially connected with the filtering inductor Lf and the two switch elements at the upper side and the lower side of each basic unit Ub perform the complementary operations. Consequently, the conduction current of each switch element is the current of the filter inductor. In a switching period T, the forward conduction directions of the switch elements Q2 and Q1 are identical to the positive direction of the inductor current. If the inductor current IL of the filtering inductor Lf is stable, the relationships between the change amount ΔV1 of the voltage Vc1 of the flying capacitor C1 and the duty cycles D1 and D2 of the two serially-connected switch elements Q1 and Q2 can be expressed by the following mathematic formulae:ΔV1=+IL×Ts×(D2−D1)/C1, (if IL>0)ΔV1=−IL×Ts×(D2−D1)/C1, (if IL<0)
The above two mathematic formulae can be rewritten as the following formula:D2−D1=sign×C1/(IL×Ts)×ΔV1,wherein sign=+1 if IL>0, and sign=−1 if IL<0).
According to the above formula, a duty cycle difference between the duty cycles D1 and D2 of the two serially-connected switch elements Q1 and Q2 (i.e., ΔD=D2−D1) is an index of controlling the voltage balance of the flying capacitor. That is, the adjustment of the duty cycle difference ΔD is related to the anticipated voltage change amount ΔV1 of the flying capacitor C1. In addition, the adjusting direction of the duty cycle difference ΔD is related to the current direction “sign” (i.e., the positive or negative sign of the inductor current IL of the filtering inductor).
As shown in the above formula, the adjusting direction of the duty cycle difference ΔD is determined according to the current direction (i.e., the positive or negative sign of the inductor current IL). For achieving the voltage balance control of the flying capacitor, the control system has to realize the direction of the inductor current IL. In other words, the current direction of the filtering inductor is an important factor influencing the charging/discharging procedure of the flying capacitor and maintaining the voltage balance of the flying capacitor.
However, the above voltage balance control method still has some drawbacks. For example, in case that the voltage is near the AC zero-crossing point of an AC/DC converter or a bidirectional power converter is operated under a light load condition, the ripple current generated by the high frequency switch element may result in the repeat switching action of the current direction. Moreover, because of the errors introduced through the measuring component, the measuring circuit and the digital sampling circuit, the control system may erroneously judge the current direction. Under this circumstance, it is difficult to control the voltage balance of the flying capacitor. Moreover, this problem limits the applications of the flying-capacitor multilevel converter.
Therefore, there is a need of providing a voltage balance control device and a voltage balance control method for a flying-capacitor multilevel converter in order to overcome the above drawbacks.